This Shared Resource began in 1991, with funding provided by the Department of Therapeutic Radiology. Its major equipment is a Cesium-137 laboratory irradiator (Mark I, Model 68A) manufactured by J.L. Shepherd and Associates, Inc. It contains two cesium-137 sources with activities of 6,000 and 100 curies each. This instrument has been purchased by the Yale School of Medicine (YSM) with funds provided by a Shared Instrumentation Grant from the National Institutes of Health (grant number 1-S10-RR04813-01; Principal Investigator, Dr. Ravinder Nath). The Cesium-137 Irradiator Shared Resource provides access to irradiations by ionizing radiation, specifically 662 keV gamma rays from the radioactive decay of cesium-137. This facility can operate at dose rates ranging from 0.01 cGy/min to 1500 cGy/min. The purpose of the Cesium-137 Irradiator Shared Resource is to provide an accurate and reliable irradiation resource for molecular compounds, cell culture, animal and human tumor cells, and small animals. Our specific aim is to keep an irradiator available 24 hrs/day, 365 days/year for any cancer researcher that requires gamma irradiation. This cancer-center core facility provides various scientists in the Cancer Center the ability to study the effects of specific radiation-induced lesions in the DNA of cells, as well as what repair pathways are involved after radiation damage, and other radiobiological mechanisms. The radiation delivered can also be used as a tool in cancer research to suppress the immune response in cell cultures or animals, e.g. by generating irradiation-induced bone marrow chimeric mice in transgenic gene targeting research experiments, and the induction of apoptosis in murine spleen cells. The Cesium-137 Irradiator Shared Resource has been instrumental in the development of 3-dimensional polymer based dosimeters for applications in radiology, as well as the investigations ofthe radiation physics and the dosimetric response of superheated emulsion gels. In hematology labs, gamma irradiation of cellular blood components is still the acceptable method for the prevention of transfusion-associated graft-versus-host disease. The Cesium- 137 Irradiator Shared Resource can also be used for the sterilization of various cell, vaccine and drug preparations. Since its inception, the Cesium-137 Irradiator Shared Resource has enabled 161 Principal Investigators (124 YCC Members) from 50 departments/sections at Yale the ability to perform a broad range of cancer research projects involving irradiation over a wide range of dose rates and doses. In the past year, 25 YCC members generated 94% of the total usage ofthe Cesium-137 Irradiator Shared Resource and participated in 6 of 7 of the YCC Research Programs.